Lumber curing process



May 13, 1958 E. R. 4GREIENHOOD LUMBER CURING PROCESS Filed May 24, 19544 Sheets-Sheet 1 4 Inverzivaf:

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May 13 1958 E. R. GREENHOOD 2,834,120

LUMBER CURING PRocEss Filed may 24, 1954 4 sheets-snaaiv 2 Esa Russeifeezzood, y 7%, e yfm Hwzegys May 13, 1958 E. R. GREENHOOD 2,834,120

LUMBER CURING PRocEss Filed May 24. 1954 4 sheets-sheet s Eisa llwseZZweeazanod, w diloafneys 4 Sheets-Sheet 4 My 13, 1958 E. R. GREENHOODLUMBER CURING PROCESS Filed May 24, 1954 United LUMBER CURNG PRGCESSElisha Russell Greenhood, Wellesley, Mass.

Application May 24, 1954, Serial No. 431,671

Claims. (Cl. 34-37) The invention relates tothe curing of green orpartly dried lumber. The present application is a continuationin-part ofthe inventors copending application Serial No. 213,106, led February 28,1951 now abandoned.

The methods commonly used for drying lumber are classified generally asair drying and kiln drying. Air drying is accomplished simply bystacking the lumber in a yard or shed for a long enough period so thatthe evaporation which takes place under natural conditions reduces theinternal moisture of the lumber to a tolerable percentage. Kiln drying,as it is presently practiced, is essentially a process of heating thelumber in a closed kiln to speed up the evaporation of internalmoisture. Although the process here described might be referred to as akiln drying process, inasmuch as a kiln is employed, it differs inseveral respects from conventional kiln drying, and is more properlytermed a curing, rather than a drying, process as it produces effectsother than mere moisture removal.

According to the most common method of kiln drying, the lumberis stackedin a kiln, and warm air is circulated through the stack. By controllingthe. humidity and temperature of the air the rate of evaporation iscontrolled. Ordinarily the drying is started at high humidity and lowtemperature, and the humidity is gradually reduced and the temperatureincreased during the drying cycle. The purpose of circulation is tomaintain uniform conditions of temperature and humidity throughout thekiln, and comparatively low circulation speeds are adequate. It isadvisable to use a high humidity at rst to avoid too rapid evaporationat the surface, and in some cases the humidity is kept high enough toprevent evaporation altogether until the lumber has become heatedthrough. These precautions are necessary to prevent case hardening, acondition resulting from too rapid drying of the surface of a boardwhile the interior is still wet. In this condition the surface hasshrunk and set under considerable tension. This condition makes a boardstili and also gives rise to such defects as checking, cracking andwarping. In spite of such precautions', some degree of case hardeningfrequently occurs, leaving the boards stit, and it is then necessary todestress the boards by restoring moisture to their surfaces. Casehardening also prevents proper drying of the interior of the board, asthe internal moisture cannot penetrate the case hardened surface. Theoptimum temperature and humidity schedule for safe drying variesconsiderably for diiferent types of wood and for different lots of thesame wood, and the control of the process requires skilled judgment, asWell as constant vigilance andfrequent testing by the operator. An errorin judgment can result in a very costly loss. i

Conventional kiln drying, although much faster than air drying, is stilla slow process, as it takes many days to kiln dry green lumber to 12%moisture content, which is about the average ultimate moisture contentfor air dried lumber, and several days to reduce the moisture contentfrom 12% to 8%, which is representative of 2,834,120 Patented May 13,1958 ice 2 the dryness required for lumber to be used for furniture,lindoor trim, and similar purposes.

Both air drying and conventional kiln drying are strictly evaporationprocesses and accomplish no more than removal of moisture from thelumber. The lumber rcrnains highly hygroscopic. Such lumber must be keptin heated storage until used or, if left outdoors for a considerableperiod, will regain high moisture content. Furthermore, because of itshygroscopicity and conse quent tendency to shrink and lswell underchanging atmospheric conditions, wood is inferior for many purposes toother materials, such as metals and plastics.

The chief object of the invention is to provide a process for curinglumber which is much fasterand more economical than present kiln dryingprocesses, which substantially eliminates the danger of case hardening,with consequent checking, cracking and warping, which can beautomatically controlled so that the results do not depend on the skilland experience of the operator, and which renders the lumbersubstantially less hygroscopic and thus more dimensionally stable thanlumber dried by conventional methods, while retaining the originalstrength and flexibility of the boards.

Another object is to provide inexpensive and efcient apparatus forcarrying out the process. The process and apparatus here described aresuitable either for curing Y green lumber or for further curing oflumber which has been previously dried by other methods.

Further objects and advantages of this invention will be apparent fromthe following description of the process in connection with therecommended apparatus shown in the accompanying drawings; but it will beunderstood that tne structural details of the apparatus hereinillustrated and described as a means of carrying-out the improvedprocess, may be varied to suitv particular conditions or installations,without departing from the essence of the invention as defined in theappended claims.

The apparatus for carrying out the process here described is illustratedby drawings, in which Fig. l is a horizontal'section through a kilnequipped with the improved lumber curing apparatus;

Fig. 2 is a longitudinal section on line 2 2 of Fig. 1;

Fig. 3 is a transverse section on line 3-3 of Fig. 1, showing the outerend of the kiln car with the lumber stacked thereon;

Fig. 4 is a transverse section at the inner end of the kiln car, showinga modification;

Fig. 5 is a transverse section of the kiln showing the inner wall of theempty kiln, complemental to the modication of Fig. 4; and

Fig. 6 is a perspective, exploded view of the modified kiln car and itscover members.

ln the form chosen for the purpose in Figs. 1-3, the lumber curingapparatus comprises a relatively small shed or other enclosure havingheat-insulated side walls 10, at least one of which is provided with adampered vent 11 in its upper portion, an insulated end wall 12, and aninsulated roof 13, erected on a foundation 14 of concrete, and having atransverse, insulated wall 15 dividing the shed into a kiln compartment16, and a blower compartment 17 having a subcompartment or control room13 defined by a partition 19. The oor of the kiln compartment preferablyhas tracks Ztl for a kiln car 21 movable into and out of the kilnthrough an insulated door 2'2.

Compartment 17 houses one or more blower units of any standardconstruction and operation, two such units being shown at 23 and 2li inFig. 1. The fans of the -blower units (not shown) being driven by motors25 and 26, respectively, suitably mounted in control room 18 andoperatively connected to drive shafts 27 and 28 passing through thepartition 19. The respectivev blower units have air intake ports 29 and30, and outlet-ducts 31 and 32 leading to delivery ports spacedvertically with respect to each other in the center of the wall 15. Saidports preferably have louvres 33 and 34, respectively,

or other air deecting vanes, for dilfusing the air driven into a tunnel(hereinafter described) in the center of the kiln 16.

The apparatus is here illustrated as employing a pair of blowers. It isunderstood, however, that a single blower of adequate power to producethe necessary volume and velocity of circulation may be centrallymounted so as to discharge into the tunnel through a single duct. Theblower or blowers must be of a capacity to deliver air or vapor into thetunnel at relatively high speed, the speed being preferably related tothe tunnel size in such a manner as to change the tunnel atmosphere atthe rate of about sixty times a minute, for the reasons to be furtherexplained with reference to the process.

A steam pipe 35 is connected to heating coils 37 which are placed in thedischarge ducts of the respective blowers to heat the kiln atmosphereentering the tunnel. ond steam pipe 36 opens directly into the ducts tosupply steam for humidifying the atmosphere, as may occasionally berequired. Steam as supplied through these pipes from any appropriatesource (not shown) and the admission of steam to the pipe is regulatedby appropriate control valves of conventional type. The respective endportions of the wall have vertical openings 39 through which air forcedinto the kiln 16 may return to compartment 17 after passing through thelumber stacks, for recirculation through the blower units.

The control room 18 is accessible through a door 40 in the end of theshed, and a control or instrument box is suitably located therein, asindicated diagrammatically at 41. 1t is understood that the valve forthe steam supply may be automatically controlled by 'a thermostaticcontrol of any conventional type.

The boards 4S are closely stacked with intervening stickers 46 on thekiln car 21 to provide two spaced stacks 47 and 48, respectively (Fig.3) on opposite sides of the car, and to dene an intermediate tunnel ormixing chamber 49 (Fig. l). The outer end of said tunnel is closed by apanel 50; its top is closed by a cover 51; and its inner end is closedby panel 52 which has openings registering with the outlets of the ducts31 and 32, respectively. Hence, air or vapor driven through said ductsis forced into the partially closed tunnel 49, thence forced laterallythrough all portions of the lumber stacks 47 and 48 to the sides of theclosed kiln, and then back to the blower compartment through the ports39, for recirculation.

It is thus apparent that operation ofthe motors Z5, 26 causes the blowerfans to drive vapor drawn into the inlets 29, 30, through the outlets31, 32 into the tunnel 49, then laterally through all portions of thestacked lumber, and thence from the kiln to the -blower inlets, as acontinuous circulating system. The circulating vapor may be heated todesired temperature by regulating the steam supply to the heating coils37, or cooled by shutting olf the steam pipe 36 and opening the cold airdampers 38. And the circulating vapor may be moistened or humidified byadmitting steam through pipe 35, as aforesaid.

The operation is the same with the modiiied apparatus of Figs. 4 to 6,where the lumber is so stacked on the car 21 as to provide a central,longitudinal tunnel which is substantially triangular in cross-section,and the compartment Wall 54 has only one vapor port 53, said port havingdeflecting vanes 61 (Fig. 5). The tunnel between the lumber stacks 55and 56 is closed at its inner end by a triangular shaped panel S7 (Fig.4) having an opening 58 adapted to register with the delivery port 53.The opposite, outer end of the tunnel is closed by a triangular panel59, and a narrow cover 60 closes the top of the tunnel when applied tothe upper ends of said panels after lumber is stacked against theinclined edges thereof. This modeof stacking may be preferable when onlyone blower outlet is provided near the bottom of the kiln chamber,

A secas indicated in Fig. 5, to ensure substantially uniform lateral airow through the stacks.

In either stack arrangement, the panels closing the top and ends of thetunnels may be omitted if the lumber stack extends almost to the roofand the end wall of the kiln, so that the incoming atmosphere issubstantially conlined to the tunnel and forced laterally through thestacks.

The improved curing process is performed by means of the apparatus justdescribed in the following manner:

The lumber is stacked in the kiln, and the blower is turned on. At thesame time suiiicient steam is supplied to the coils 37 to raise thetemperature of the atmosphere in the tunnel to F. in about an hour. Thisperiod may be termed the heating stage, and the chief pur- 2 pose of therapid circulation is to carry enough total heat to the lumber so that itbecomes heated through. When the sap in the lumber has reached atemperature of about 140 F., the phenomenon of sap removal which isobserved during the next stage, or curing stage, begins to occur.

During the next stage, which may be termed the curing stage, the heatingof the entering atmosphere by means of heating coils 37 is continued tomaintain the temperature of the kiln atmosphere above 140 F. Preferablythe thermostat is set for 212 F. to 220 F. and the initial heating rateis continued until the atmosphere in the tunnel reaches this limit. Thehigh velocity circulation is continued throughout the curing stage whichmay last from about ve hours to about three days, depending on the typeand size of the boards to be processed.

When the lumber has been cured to the desired moisture content, the heatis shut otf. The lumber can then be removed from the kiln.

The process has been described as being accomplished in two stagesbecause of the fact that the sap removal phenomenon, which is about tobe discussed, begins to be observable when the temperature of the tunnelatmosphere reaches about 140 F. In actual practice the operator merelysets a thermostat or automatic temperature control at 220 and turns onthe Iblower system at the beginning of the run. If the capacity of theheating system is such as to raise the temperature of the tunnelatmosphere to 140 F. in about an hour, no further manipulation of thecontrols is needed. The vent 11 may be opened during the process toremove moisture from the atmosphere more rapidly.

By this process lumber may be cured and dried to any desired degree muchmore rapidly than by anyconventional kiln drying method, as evidenced bythe following table, showing typical curing runs for various types andsizes of lumber:

Final Specie Thickness, Starting Moisture Length inches MoistureContent, of Run,

Content percent i hrs.

1 S-l0 6 l 5-7 1 12-15 24-30 .l5-18 72 1 d0. 48 do 8 48 2 d0 8 48 Boardscured by the process are exible, thoroughly pre-shrunk, and remarkablyfree from case hardening and other defects, and compare favorably to airdried lumber in structural strength. Furthermore, the boards are highlyresistant to reabsorption of moisture. Boards left outdoors for longperiods have been found to reabsorb only 2% to 3%, whereas boards driedby conventional methods reabsorb so much moisture that they must bere-dried after similar exposure. When this process is employed, the needfor maintaining large and costly indoor storage facilities is thuseliminated. Due to their low hygroscopicity, the boards cured by thisprocess have a high degree of dimensional stability, a quality which isof great value in almost any end use for which wood is a suitablematerial.

The velocity of circulation of the atmosphere immediately surroundingthe lumber employed in this process is considerably higher than thecirculation speeds commonly used in conventional kiln drying operations.A velocity suicient to circulate tunnel atmosphere 60 times per minuteappears to be about the optimum from the standpoint of curing time andeconomy. This radical increase in the circulation speed gives rise toeffects other than mere evaporation of moisture. While the kiln is inoperation a considerable amount of resinous substance is deposited onthe walls and floor ofthe kiln, indicating that much of the sugar andresin content of the sap is removed from the lumber along with themoisture. No humidication of the atmosphere is required except thatsteam may be admitted at the beginning of the operation through pipe 35if the boards are Very dry on the surface. The same thing can beaccomplished by wetting down the boards before the run is started. Thisinitial wetting serves to establish contact of the surface moisture andthe inner moisture to facilitate the starting of the sap ow. When thesap removal has started the moisture is drawn to the surface sulicientlyrapidly to keep the surface wet and prevent case hardening.

Two conditions are necessary for-successful application of the curingprocess:

(l) The internal temperature of the lumber must be maintained at atemperature above about 140 F. This minimum temperature is not an exactlimit but appears to be the point at which sap removal begins to occurto a noticeable degree under average conditions. The temperature may beconsiderably higher than 220 F., but temperatures in excess of 240 to250 may cause discoloration of the wood.

(2) The velocity of circulation of the kiln atmosphere over the woodmust exceed a certain lower limit, for example, a speed high enough tochange the atmosphere immediately surrounding the boards about 40 timesper minute. This limit is merely approximate and may very according tothe type, size, and condition of the wood, as well as the temperaturemaintained. The lower limit of circulation speed may be said to be theminimum speed at which noticeable amounts of visible moisture and/or sapsolids appear in the kiln atmosphere when the temperature is atapproximately 140 F. or over. The process can be used to advantage evenbelow the speed at which sap solids are removed, as the drying time isshorter, as long as the speed of circulation is sufficient to removeinternal moisture by dynamic or rnechanical means. The presence of sapsolids may be readily determined by observing condensate from the kilnatmosphere on any cool surface. lf the condensate is sticky, this is anindication that removal of sap solids is occurring. From the standpointof operability of the process there appears to be no upper limit for thecirculation speed. It is a practical matter, however, that anyadvantages resulting from shortened curing time must be weighed againstthe increase in power requirements, as the power required increases outof proportion to the gain in speed. it is obvious, also, that excessivespeeds will result in undue stress on the kiln structure and on thelumber itself. Speeds suiicient to change the atmosphere around theboards at rates or frequencies varying from forty to ninety times perminute have been successfully employed.

The blower capacity for these speed requirements may be calculated onthe basis of the volume of the tunnel, as the ends and top of the tunnelare conned so that substantially all of the atmosphere entering thetunnel is discharged through the stacks. For example, for a tunnel 31/2feet wide by ll feet high by l2 feet long, the volume would be 462 cubicfeet. To move this volume sixty times per minute the lower must be -ableto deliver 4 27,720 cubic feet per minute (calculated at atmospherepressure.)

The physical phenomena which occur in the processare not entirelyunderstood. It is believed, however, that the following discussionexplains, in part, what occurs:

The heating sets up molecular agitation and consequent internal pressurein the sap, causing the sap to expand and migrate to the surface, andalso redces the viscosity so that the sap will flow more readily. Sap owtoward the surface is induced by aerodynamic forces, such as the dynamicpressure differential set up between one side of the board and the otherby the rapidly moving air stream. This pressure differential causes thesap to be blown or sucked from the lumber. The friction of the rapidlymoving atmosphere may also serve to tearlotf particlesv of sap andmoisture from the surface. The high velocity of circulation not onlyproduces these dynamic effects but also serves to carry enough heat tothe lumber to maintain the molecular agitation and low viscosity of thesap.

From'the table of representative curing time, it is apparent that thisprocess is much quicker than conventional kiln drying. For example,green lumber can be cured to 12% moisture content in 24-30 hours, anoperation which ordinarily takes five or six days by kiln drying. Asmall kiln can thus handle as muchlumber in the same time as one aboutve times as large. The smaller space required and the reduction inhandling and storage costs makes the overall cost of the process lower,even though the power and steam lconsumption for the same size kiln maybe higher when this process is used.

The resulting product is superior in quality and in resistance to waterabsorption as previously discussed, and has excellent strength andflexibility.

What is claimed is:

1. The process of curing a stack of boards arranged in spaced courses ina kiln which comprises circulating in a horizontal path a gaseousatmosphere through a horizontally disposed stack at a velocity in excessof 200 feet per minute so as to change the atmosphere immediatelysurrounding the lumber at a frequency between 40 and times per minute,while supplying heat to the circulating atmosphere at a rate suitable toraise the temperature of the lumber to F. in about an hour, thereafterheating the atmosphere at the same rate to a temperature in the range of212-220 and maintaining the atmosphere at said temperature whilecontinuing the circulation at said velocity.

2. The process of curing lumber which comprises arranging boards at andhorizontally in spaced courses in two laterally spaced stacks in a kilnto form a substantially conined tunnel between the stacks disposedlongitudinally of the boards, and blowing a gaseous atmospherehorizontally into the tunnel and thence laterally through the stacks andthence horizontally back to the tunnel at a speed in excess of 200 feetper minute so as to change the atmosphere immediately surrounding thelumber at a frequency between 40 and 90 times per minute, while` heatingthe atmosphere to maintain it at a temperature of at least 140 F., andcontinuously recirculating the same atmosphere, including materialsextracted from the lumber, through the stacks.

3. The process of curing lumber which comprises continuouslyrecirculating in an unimpeded horizontal path agaseous atmosphere overhorizontally disposed lumber at a velocity in excess of 200 feet perminute so as to change the atmosphere immediately surrounding the lumberat a frequency between 40 and 90 times per minute, while maintaining theatmosphere at a temperature in the range between 140 F. and 250 F.

4. A process as described in claim 2, which includes heating theatmosphere to the specified temperature at the region of its dischargeinto the tunnel.

5. The process of curing lumber, which comprises arranging boards inspaced horizontally disposed courses 7 in two spaced stacks in` a kilnto form a substantially confined tunnel between the stacks disposedlongitudinally of the boards, and continuously circulating in ahorizontal path and at a velocity which changes the atmosphereimmediately surrounding the boards at a frequency between 40 and 90times per minute a gaseous atmosphere into the tunnel through the stacksand back to the tunnel, While heating the atmosphere to maintain it at atemperature of at least 140 F., all the circulation taking placesubstantially in planes parallel to the boards.

References Cited in the le of this patent UNITED STATES PATENTS OTHERREFERENCES Thelen: Kiln Drying Handbook, U, S. Dept. oAgriculture,Bulletin No. 1136, May 1929, pp. 72 to 77.

Drying by Means of Superheated Steam and the Kiln Drying of Softwoods,U. S. D. A., Forest Products Laboratory Report 702, revised August 15,1931.

The drying Rate of Sugar Maple as Affected by Relative Humidity and AirVelocity, U. S. D. A., Forest Products Laboratory Report R1264, December1940 13 1"' pages.

Why the Drying Time of a Kiln Load of Lumber is Affected by AirVelocity, U. S. D. A., Forest Products Laboratory Report R1269, .lune1941, 5 pages.

Mettet: The Wind Tunnel Lumber Dryer, West Coast Lumbermans Association,Research Bulletin No. 6, April 1946, pages 1 to 8.

A Small Lumber-Drying Unit Employing a Portable Crop Drier for Heat andAir Circulation, U. S. D. A., Forest Products Laboratory Report R1799,June 1952,

20 21 pages. Pages 4, 5 and 12 relied on.

